1. Field of the Invention
The present invention relates to a cleaning composition for a molding machine and a cleaning method. More particularly, the present invention is concerned with a cleaning composition for a molding machine which comprises a specific graft polymer and a thermoplastic styrene polymer, and a method for cleaning the interior of a molding machine having residual molding resin retained therein, in which the above-mentioned composition is utilized. The cleaning of the interior of the molding machine is required at the time of, for example, changing the type or color of a molding resin. Efficient, complete cleaning is attained by the present invention.
2. Discussion of Related Art
Generally, molding machines, such as an injection molding machine, an extrusion molding machine and a blow molding machine, are used to obtain a shaped article of a resin. When it is intended to change the type or color of a resin to be molded (hereinafter frequently referred to as "molding resin") in which various additives, such as a pigment and a dye, are often incorporated or when it is intended to shut down a molding operation, it is necessary to stop the molding machine. In the stopped molding machine, usually, a portion of the molding resin is retained. Therefore, if a molding resin of different type or color is processed by the molding machine without removing residual molding resin retained therein, a molded article is caused to have poor appearance and poor properties due to the mixing of the residual molding resin into the molded article. Further, even if the same molding resin is employed, when the molding is conducted without removing residual molding resin retained therein after the molding machine has been stopped for a certain period of time, a molded article has also poor appearance and poor properties due to the mixing of the residual molding resin, which has been decomposed or denaturized, into the molded article.
In the art, the residual molding resin retained in a molding machine has conventionally been removed by the following methods. One method is to disassemble the molding machine and manually clean the interior of the molding machine. Another method consists in charging a molding machine having residual molding resin retained in the interior thereof, with a resin to be used for a subsequent molding operation and operating the molding machine to pass the resin through the molding machine to thereby remove and withdraw the residual molding resin. A further method consists in charging a molding machine having residual molding resin retained in the interior thereof with a cleaning composition, operating the molding machine to pass the cleaning composition through the molding machine to thereby remove and withdraw the residual molding resin, while causing a part of the cleaning composition to be retained as a residual cleaning composition in the interior of the molding machine, charging the molding machine having the residual cleaning composition retained therein with a fresh molding resin, and operating the molding machine to replace the residual cleaning composition with the fresh molding resin.
The above-mentioned known cleaning compositions are (1) a composition (semi-melt type) comprising an acrylic resin having a superhigh molecular weight and, contained therein, a foaming agent, kaolin and calcium carbonate, (2) a composition (complete melt type) comprising a resin containing a surfactant, and (3) a composition (blend type) comprising a metallic soap containing a foaming agent, which is adapted to be blended with a resin before use in the cleaning of a molding machine.
However, all of the above conventional cleaning methods have inherent drawbacks. That is, the manual method in which the molding machine is disassembled, has drawbacks in that the workers' time is gravely wasted, thereby causing productivity to be low. Further, in this manual method, residual molding resin is likely to remain in some particular regions of the molding machine, for example, on the inner wall of a cylinder, where disassembly of parts is difficult for a structural reason.
In the method in which the removal of residual molding resin is performed by the use of a resin to be used for a subsequent molding operation, the cleaning effect of the resin is so low that a large amount of the resin is wasted, and that a long period of time is consumed for cleaning, because such a resin is generally suitable for molding but not for cleaning. Further, some types of residues retained in a molding machine, such as thermal decomposition products of rubbers, cannot be easily removed by this method.
In the method in which the removal of residual molding resin is performed by the use of a cleaning composition, all of the known cleaning compositions have serious drawbacks, which render it difficult to put the conventional cleaning compositions in practical use. That is, in the case of the semi-melt type cleaning composition (1), the composition itself is likely to remain as a residue in the interior of a molding machine, which residue is likely to get mixed into a fresh molding resin, thereby causing the final shaped article to have poor properties. In the case of the complete melt type cleaning composition (2), it has poor cleaning effect, and it is also likely to remain as a residue in the interior of a molding machine, causing the satisfactory removal of the residue to be troublesome and time-consuming. In the case of the blend type cleaning composition (3), it usually needs to be blended with a molding resin before use. This composition also has poor cleaning effect, and the prior blending with a molding resin is disadvantageous from the viewpoint of production efficiency. In the case of this blend type cleaning composition, best results are obtained when the type of a molding resin to be blended with the cleaning composition in the prior blending is the same as that of the residual molding resin to be removed. Accordingly, when a wide variety of molding resins are employed in a molding workshop, this disadvantageously makes it necessary to prepare a wide variety of cleaning compositions.
In the method in which the removal of residual molding resin is performed by the use of a cleaning composition, the residual cleaning composition remaining in the interior of a molding machine must be replaced with a fresh molding resin before molding the fresh molding resin, as mentioned above. Therefore, the cleaning composition must have not only high removing effect on a molding resin employed in the molding prior to the cleaning with the cleaning composition, but also high replaceability with a fresh molding resin to be employed in the molding subsequent to the cleaning with the cleaning composition. However, all of the conventional cleaning compositions are unsatisfactory not only in the cleaning effect but also in the replaceability with a fresh molding resin.
For example, when a molding machine having been used for molding a styrene resin is cleaned by the use of a blend type cleaning composition (which has been blended with a styrene resin) and used for molding an olefin resin, it is generally observed that replacing of the cleaning composition with the olefin molding resin is difficult, thereby causing a large amount of an olefin molding resin and a long period of time to be consumed for the replacing of the cleaning composition. The similar observation is made when a molding machine having been used for molding an olefin resin is cleaned by the use of a blend type cleaning composition (which has been blended with an olefin resin) and used for molding a styrene resin.
Styrene resins and olefin resins are plastics which are most frequently employed for molding, and hence it is frequently encountered that a molding machine having been used for molding an olefin resin must be cleaned for a subsequent molding of a styrene resin, and that a molding machine having been used for molding of a styrene resin must be cleaned for a subsequent molding of an olefin resin. Therefore, there has been a strong demand in the art for a cleaning composition capable of exhibiting not only excellent removing effect for residual molding resin retained in the interior of a molding machine but also excellent replaceability with a fresh molding resin to be used for subsequent molding operation.